This invention relates to an apparatus for the vacuum evaporation of volatizable solids and their vapour deposition on a continuously moving substrate.
The idea of producing photographic materials by vacuum evaporation of silver halides has been known for a substantial time. Photographic materials prepared in this way have, in principle, considerable advantages over conventional light sensitive emulsion-coated photographic materials, namely:
1. Simple and fast processing owing to the absense of a binder;
2. High optical homogeneity reducing Raleigh scattering in the recording medium and allowing an image to be obtained which is particularly sharp;
3. A photographic material is obtained which is always thinner than the depth of focus of objectives when employing a high aperture number so that distortion of the projected image in the volume of the photographic layer is avoided;
4. High sensitivity to low energy ionizing particles and short wave ultra violet light.
These advantages are of particular value in the production of special photographic masks for use in production of microelectronic components or in systems for optical storage of information.
In spite of these advantages and the many attempts hitherto made to improving manufacturing techniques no photographic material produced by vacuum evaporation of silver halides has hitherto been entirely satisfactory. None of the methods for the production of photographic materials is entirely reproducible. This is believed to be due to the inclusion of impurities in the silver halide layers, which influence uncontrollably the properties of the silver halide layers and is generally unavoidable with apparatus hitherto employed.
Attempts have been made to reduce the impurity content of vacuum deposition-produced photographic materials and produce products with reproducible sensitivity. Thus for example in British Patent Specification No. 1,150,626, a substrate, onto which silver bromide is deposited, is cooled to -50.degree. C in order to prevent the evolution of volatile substances from the substrate, for example paper, surface contaminating the deposited silver bromide and influencing undesirably its sensitivity. In British Patent Specification No. 1,166,999 evaporation of silver bromide is carried out from a silver crucible or from a crucible of very pure, mechanically strong graphite, and the walls of the vacuum chamber employed are covered with a substance which does not react with the silver bromide, for example silver, nickel, Monel metal, glass or plastics material free from volatilizable constituents.
It is, however, well known that pure silver bromide crystals are completely insensitive and do not form, when exposed to light, a developable latent image. Reference is made here to British Patent Specification No. 1,154,741. The photographic sensitivity of silver halides is in fact due to the sensitizing action of different impurities (metal salts, sulphur compounds, etc.). The fact that the silver halide layers obtained by the methods described in British Patent Specifications Nos. 1,150,626 and 1,166,999 are light sensitive is definite proof that uncontrollable contamination of the material nevertheless occurs during the production thereof. Obviously apparatus employed hitherto cause the uncontrollable inclusion of impurities during the process of evaporation and deposition of silver halides on a substrate. Not even the method described in British Patent Specification No. 1,154,741 for doping ultra-pure light insensitive silver bromide, in which silver bromide is first vacuum deposited on a moving substrate will, in fact, yield a light-insensitive silver bromide layer on the substrate and sensitizing by subsequent doping is not reproducible.
It would therefore seem that to prepare silver halide layers having fully controllable and reproducible properties, apparatus must be devised for enabling vapour deposition of highly pure photo insensitive silver halide layers having not photographic sensitivity at all to take place. Such layers can then be subjected to deliberate and controllable sensitization to provide a material having reliable and reproducible properties.
A common feature of all the hitherto forms of apparatus is the direct contacting of the heating element of the crucible with the silver halide melt. Since a silver halide is a good ionic conductor when in the molten state, the silver halide is caused to undergo electrolytic decomposition by the current flowing through the heating element. Since thermal decomposition of silver halides is autocatalytic, the formation of electrolytic silver therefrom enhances evolution of highly reactive halogen gas, which is believed to be partially responsible for the contamination of the silver halide layers.